Guide
What Is Inside a uPVC Door Lock Mechanism, and What Goes Wrong
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Naming the Parts of a uPVC Door Lock
The first thing to know about a uPVC door lock is that it is three parts in one place, and they are regularly mistaken for each other. The locking points are carried by a strip in the door edge. That strip is driven by a gearbox. The key goes into a cylinder, which passes through the gearbox area. All three get called the lock, and that loose word is why quotes can vary so much for the same door in Hastings or anywhere else.
Locking Points and the Strip in the Door Edge
The multipoint mechanism is the strip itself, and its faceplate is the flat metal strip-cover you see with the door open. The locking points differ in how they move. A hook pivots out like a claw and drags the door towards the frame. A roller turns on its stud and can be adjusted to change how firmly the door closes. A mushroom has a widened head whose shape resists the door being forced apart. A pinbolt and a deadbolt travel straight out, with the deadbolt as the heavier relative. Fixed in the frame, a keep receives each point.
The Cylinder, the Gearbox and the Handle
The handle is attached to a spindle, the square shaft through the door. The follower is the gearbox's square socket for that shaft. Inside the gearbox, the turn becomes a push along the strip. The euro cylinder is the barrel for the key, passing through the mechanism as a separate part, and its cam is what turns the gearbox when you lock with the key.
The Four Kinds of Mechanism in uPVC
Parts suppliers divide uPVC mechanisms four ways, into the full lock, the gearbox centre case on its own, the overnight lock and the auto slam shut, and homeowners rarely meet two of those names. UPVC Spares 4 Repairs lists its mechanism types this way, so these are trade categories for sorting stock, not a standard anyone certifies. Each label answers two questions together: what part is being bought, and what the door does as it closes.
Gearbox Centre Case and Full Lock
Order a full lock and you get the whole strip, which must match the door completely. Order a gearbox centre case and you get the driving unit alone, which suits only mechanisms designed for it to be removed.
Overnight Lock and Auto Slam Shut
A door with an overnight lock rests on its latch after closing; the handle has to be lifted and the key turned to lock it. A door with an auto slam shut engages its locking points by itself as it closes.
| Type | Behaviour | Repair |
|---|---|---|
| Full lock | Follows the fitted strip | Strip replaced in full |
| Gearbox centre case | Unchanged | Centre case replaced alone |
| Overnight lock | Latch until handle and key | Overnight replaced with overnight |
| Auto slam shut | Self-locks on closing | Slam shut replaced with slam shut |
If a slam shut door has locked behind you, that is a separate job:.
The Usual Order in Which Things Fail
The gearbox fails; most other parts simply wear or move out of alignment. That is the short version of what goes wrong with a uPVC mechanism, and the order is no accident, because the gearbox carries the effort of every lift and nothing else in the door edge works as hard. Look back before most failures and you find weeks of stiffness, which means a door that has become hard work is already warning you.
| Part | Symptom |
|---|---|
| Gearbox | Handle slack, or seizing before full travel |
| Follower and spindle | Vague, loose handle, often the step before failure |
| Rollers and keeps | Door needs a shove; points engage partially; this is alignment |
| Euro cylinder | Key trouble with a handle that works |
| Faceplate screws | Rare, though a loose one makes other faults feel worse |
Those symptoms are diagnosed properly in three places: a door that refuses to lock, the door that will not open at all, and a key that will not turn at all.
How a Mechanism Is Matched and Identified
Four measurements and a locking point count decide which replacement mechanism fits a uPVC door, and a single error among them produces a part that will not go in. The quickest shortcut is on the door itself. Most faceplates carry the manufacturer's name stamped into the metal, and once you have it, the search shrinks dramatically. Treat that name as a label for identification only, never as a judgement on quality.
Four Measurements
These four come from the filters UPVC Spares 4 Repairs uses, with the Home Secure changeover guide giving matching definitions for the first two:
- Backset: from the front of the faceplate to the centre of the cylinder cut out
- PZ, also called spindle to key: from the centre of the cylinder to the centre of the spindle, stocked from 48mm up to 117mm
- Faceplate width: stocked from 16mm to 45mm
- Screw centres: commonly 215mm or 240mm
Those spans show why guessing fails: PZ alone covers a wide range. The strip's overall length, end to end, must also agree.
Counting Locking Points
Lift the handle with the door open and note each point's shape. The supplier lists hooks at anything from none to five, rollers at none, two or four, mushrooms at none or two, pinbolts at none or two, and deadbolts from none to three. Two doors that look identical from outside can carry very different strips.
Checklist: stamp, backset, PZ, faceplate width, screw centres, length, and a photo of every point out.
Keeping a Mechanism Working
Lock with the handle lifted as far as it will go, every time. That habit does more for a gearbox than anything else, because a gearbox pushed only half way still carries the load while stopping short of the end of its travel. Four further points complete the routine.
Edge, not keyhole
the hooks, rollers and bolts are what move, so lubricate them with a product designed for locks.
Clear keeps and threshold
obstructions there make each locking point push harder than it should.
Early stiffness
respond as soon as it shows, since it is the notice that comes before most failures.
Light handles
heavy items hung on the handle keep the spindle and gearbox under constant load.
Expect a door to behave a little differently between the seasons too. And keep expectations honest: looking after a mechanism postpones the day the gearbox fails, but it cannot stop that day coming, because the part is built to wear.
Gearbox Alone, or the Whole Strip?
It is the mechanism, and not whoever is quoting, that settles whether the gearbox can come out on its own. Three points, taken in order, explain how.
- On a mechanism with a removable centre case, the gearbox is changed and everything else, hooks and faceplate included, stays in the door.
- On a mechanism without one, a gearbox fault means the full strip, matched on every one of the four measurements and every locking point used in the UPVC Spares 4 Repairs filters.
- The Home Secure changeover guide states that full multipoint locks are too long to ship and have to be matched and bought in person, while gearboxes are posted.
So a gearbox can often be sourced quickly, and a full strip usually means somebody at the door comparing the old strip in their hand with what is available. If you are told your door needs the whole strip, those are the reasons that decide it, and they are worth asking about.
Getting Your Mechanism Identified
Everything above points to one thing: the mechanism has to be identified before anything else happens. So call B.T Locksmiths Hastings, read us the faceplate stamp and describe the door edge with the handle lifted, and we can usually say which mechanism it is and whether the gearbox is replaceable on its own.
Before you call, gather: the stamp; backset, PZ, faceplate width and screw centres if you have them; and a description of what the handle is doing.
At no hour is there a call-out charge. Calls are taken 24 hours a day, 365 days a year. The price comes on the call, worked out from your description, and if what is found on site changes it, you know before anything is done.
Describe It and Get It Identified: 01424 571056
If a call is not convenient, call or message B.T Locksmiths Hastings instead, and the outcome may well be lock repair rather than a full replacement.
uPVC Mechanism Questions
My handle has gone floppy and the door will not lock. Is that the gearbox?
The gearbox is often the likely explanation for those symptoms. However, the description alone does not establish that it has failed. Before choosing a replacement, have a qualified locksmith examine the mechanism. Treat the loose handle and failure to lock as reasons to investigate, rather than proof of a particular fault.
Can I just replace the gearbox?
The centre case must be removable for the gearbox to be replaced on its own. Do not assume that a suspected gearbox fault means a separate replacement is possible. First establish the mechanism's construction, with a qualified locksmith's help if needed, then choose the appropriate next step.
Is the cylinder part of the mechanism?
The cylinder is separate, so the answer is no. It helps to use the two names distinctly when asking about repairs. Before agreeing to a replacement, establish whether the discussion concerns the cylinder or the mechanism; one label should not be taken to mean the other.
How do I find out which mechanism I have?
Gather the four measurements alongside the maker's name and locking-point count. Find the manufacturer stamped on the faceplate, and do the counting with the handle raised. These are the details to have ready when identifying the fitted mechanism, so record them in one place.
Why can my door not be left on the latch?
The door may use a slam-shut arrangement that automatically throws the locking points on closing. If so, that explains the difficulty leaving it on the latch. This is a probable explanation, not a confirmed identification; a qualified locksmith can establish which mechanism is actually fitted.
Why does a full strip take longer to source than a gearbox?
Length is the reason given for the slower sourcing of a complete strip. Home Secure says these parts are too long to send, so they are matched and purchased in person. The time involved in doing that accounts for the difference when compared with sourcing only a gearbox.
How long should a uPVC mechanism last?
There is no fixed lifespan for a uPVC mechanism. Its age alone does not provide a replacement deadline. If the concern is whether a particular mechanism needs changing, have a qualified locksmith assess it. Avoid treating an assumed service life as a reason to replace it.